Power line conditioning systems are in common use to provide compensation for and correction of voltage sags and drops in the power provided from utility power lines to a critical load. Momentary voltage disturbances such as sags and swells, if not compensated, can result in significant financial losses for industrial power users because of potential shutdown of voltage-sensitive production equipment in the middle of critical processes, with resulting scrap, equipment damage, and lost production. Voltage sags and swells are a widespread, inherent and inevitable problem in any large-scale power distribution network. Various systems and techniques for correcting voltage disturbances have been developed and are commercially available at cost levels that generally depend on the level of protection provided. In particular, uninterruptible power supplies (UPS) are widely used and commercially available over a wide range of backup power requirements. UPS systems offer comprehensive power protection but are also relatively costly and maintenance intensive. As an alternative to full UPS systems, dynamic voltage sag correctors have been developed which utilize residual grid voltage and capacitors as energy sources rather than expensive, high maintenance, long term energy storage devices such as batteries and flywheels. Such dynamic sag correction systems are smaller, less costly, and have lower maintenance requirements than conventional UPS products, but nonetheless provide power protection for periods of time sufficient to correct more than 90% of the voltage sag and drop events which are likely to be encountered by most utility customers. Such voltage sag correctors are described in U.S. Pat. No. 6,118,676, entitled Dynamic Voltage Sag Correction.
Some types of loads which are protected by a UPS or other power protection device are capable of delivering real power back to the power protection device. One example of such a load is an electric motor, which can effectively function as a generator under certain conditions. This “regenerative” power is delivered back to the energy storage device of the power protection equipment. For cost and reliability reasons, most commercial power protection equipment utilizes a rectifier having uncontrolled rectifying devices rather than active switches so that the regenerative power cannot be transferred to the utility power lines. Conventional battery based UPS systems are often not well suited to handling regenerative loads because batteries generally are limited in the rate at which they can absorb energy. Some flywheel based UPS products can handle regenerative loads but are also more expensive and bulkier than battery based systems. For voltage sag correctors having capacitor energy storage, the regenerative power will cause the DC voltage across the storage capacitor to rise. This can potentially lead to a DC bus over-voltage condition which requires that the inverter of the device be shut off before damage to the inverter components can occur, with the consequence that sag correction is not available under these regenerative load conditions. An over-voltage condition can also occur on the DC bus when voltage spikes or transients on the utility lines are passed by the rectifier to the DC bus. Similar problems are encountered in AC power converters which supply their inverter from a DC bus across which a DC bus capacitor is connected. One solution to this problem has been to connect a power resistor and a semiconductor switch in series across the DC bus. Under regenerative load conditions or voltage spikes from the utility lines, the DC bus voltage rises until a threshold is reached, at which point the semiconductor switch is turned on and current is drawn from the DC bus through the power resistor. The resistor is sized such that, when applied, it draws charge out of the DC bus capacitor at a higher rate than the charge provided by the regenerative power or voltage spikes. The DC bus voltage thus begins to fall until a lower threshold is reached and the semiconductor switch is then turned off. The cycle repeats itself as required such that the net charge delivered to the DC bus is zero, thus keeping the DC bus voltage from rising to damaging levels. In the process, the power resistor must dissipate the regenerated energy as heat. This approach to controlling the DC bus under regenerative load conditions requires a relatively high power rated resistor which must be packaged so that it is capable of dissipating a significant amount of heat without affecting other system components, thus increasing the relative cost and size of the overall device.
AC motor drives and series connected sag correctors may utilize full bridge and half bridge inverters. In some inverter configurations, the DC bus capacitor is split and implemented as two separate capacitors connected together in series across the DC bus lines, with a DC bus common line connected between the two capacitors. It is possible under certain asymmetrical load conditions (a condition of some finite DC load current) that one half of the DC bus delivers charge to one of the capacitors while the other half of the bus collects charge from the other capacitor. The capacitor that collects charge is subject to the same over-voltage condition as discussed above, even though the total voltage across the DC bus lines might not exceed its rated maximum threshold.